1. Field of the Invention
This invention relates generally to fuel assemblies for nuclear reactors, and more particularly to a top nozzle assembly in which the adaptor plate can be quickly and easily installed and removed from the fuel assembly.
2. Background Information
The fuel for a pressurized water nuclear reactor is encased in long Zircalloy tubes forming fuel rods. A number of the fuel rods are supported in arrays, for instance 14.times.14 or 17.times.17, by a series of spaced metal support grids joined together by thimble tubes to form fuel assemblies. A series of such fuel assemblies are arranged in parallel groups to make up the core of the pressurized water reactor.
When fuel assemblies are in operation, fuel rods may become damaged requiring removal of the rod from the assembly. Removal of the rod requires that the fuel assembly be removed from the core to a separate location. The top nozzle of the fuel assembly is then removed and the damaged fuel rod is replaced. The top nozzle is reinstalled and the assembly is returned to the core. Each of these operations is performed under water with specially designed tooling, thus increasing the difficulty of the operation. The nozzle removal is often performed on a critical path during a refueling outage, hence, rapid reconstitution is vital.
The top nozzle typically includes an adaptor plate having flow openings which control the flow of reactor coolant upward through the fuel assembly and other openings which accommodate the thimble tubes. Currently available removable top nozzles include a type in which slotted inserts lock into the nozzle adaptor plate. The insert has a formed detent which locks into a circumferential groove in the adaptor plate. With the insert engaging the groove in the adaptor plate, a lock tube is inserted from above. This lock tube fixes the insert in place and completes the attachment of the removable top nozzle to the fuel assembly.
Although this arrangement does provide access to the fuel rods, several problems of have been identified during reconstitution efforts. First, top nozzle removal is difficult because forces required to disjoint the nozzle from the assembly are occasionally excessive. The difficulty in removing the top nozzle results in an increased possibility of damage to the assembly, and usually a higher cost.
Second, once the top nozzle is removed, the inserts are left exposed to possible damage from rod replacement tooling. Although damage of the inserts from rod replacement tooling is rare, the possibility of damage to the inserts increases the amount of care taken while replacing the rods. This inevitably results in increased time required for reconstitution.
Third, once the lock tubes are removed they must be stored and inspected. Handling these tubes with remote tooling provides the possibility that one of the tubes may be damaged, dropped or lost. The removal, handling and inspection operation all increase the time required for reconstitution.
Alternative arrangements for removable top nozzles include some designs in which the components of the joint are captured and either remain with the adaptor plate or the thimble tube when the adaptor plate is removed. Unfortunately, the currently available removable nozzle assemblies of this type require expensive components. Also, some of them are not readily adaptable to all types of fuel assemblies. For instance, in some fuel assemblies the joints between the Zircalloy thimble tubes and the stainless steel grids which cannot be welded, are made by bulging of the tubing. The bulging operation affects the length of the tubing, which as a result cannot be precisely determined. Hence, the joints between the thimble tubes and the adaptor plate must accommodate for this imprecision in tube length. As mentioned, the available quick release connections are either not compatible with this arrangement or require expensive parts.
There is a need, therefore, for an improved quick release removable top nozzle assembly.
There is a more specific need for such an improved top nozzle assembly which can accommodate for the imprecise length of the thimble tubes.
There is an additional need for such an improved top nozzle assembly in which the components are captured when the adaptor plate is removed, yet utilizes inexpensive easily manufactured components.